A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
In lithographic apparatus using EUV as the projection beam, transmissive optical elements cannot be used to shape the illumination beam or project the patterned beam onto the substrate because there are no suitable materials transmissive to EUV. Known illumination and projection systems for EUV radiation therefore comprise reflectors, either grazing incidence mirrors or normal incidence multilayer mirrors (also known as distributed Bragg reflectors). These types of mirrors have reflectivities significantly less than 100% so that a significant amount of energy is absorbed by each reflector. At many of the reflectors, the beam intensity is not uniformly distributed across the reflector leading to non-uniform heating thereof. Even though the substrate of a reflector may be made of a material, such as Zerodur™ or ULE™, with a very low coefficient of thermal expansion (CTE), the non-uniform heating may lead to undesirable distortions of the substrate and the reflective surface of the reflector.
An analogous condition, known as lens heating, occurs in lithographic apparatus using DUV as the exposure radiation. Various methods have been proposed to deal with lens heating in DUV lithographic apparatus, including: active cooling by flushing the lens systems with temperature controlled gas; heating parts of lenses that are not heated by the beam to ensure a uniform temperature distribution in the lens; and mechanically distorting lenses using Lorenz actuators mounted to the edges of the lenses. These approaches are not, however, easily transferred to EUV reflectors. Because an EUV optical system is maintained in vacuum, the reflective surfaces of the reflectors cannot be cooled by gas whilst active cooling of the rear surfaces is not effective in eliminating temperature variations and may introduce unacceptable mechanical disturbances. Applying additional heat to ensure a uniform heat load on a reflector is undesirable due to the difficulties of cooling optical systems in vacuum. It has been found that available actuators are not capable of applying sufficient force to a typical EUV reflector to correct its shape.